FW: DVB-T Dongle

[William Fenn]

Not Really.  COFDM was much better around town where there is a lot of
multipath.  Out side of the city ATSC was better.  The worse problem for
both was distortion in the receiving end due to folks wanting to use
amplifiers on their antennas.

 

I spoke with a number of people during the transition from NTSC to ATSC who
lived across the river in Fairfax, Arlington and Alexandria who had bought a
multi element antenna, then added a preamp and couldn't receive Ch. 48
(Virtual Channel 4).  When they bypassed the preamp things were better.
They wanted to receive the Baltimore stations and that was the reason for
the preamp. 

 

We also got a large number of calls from out in Culpepper, Delaplaine and
Paris, VA from people who used to receive Ch. 4. They were using outside
antennas and I suspect what looked good (with fine grain noise) on Ch. 4
(NTSC) became a victim of BER and the cliff effect on Ch. 48 (HDTV).

 

I used to check each complaint out using Terrain Analysis software to see
what the path to their location looked like.  In the majority of cases 30 to
35 miles out I don't think COFDM would have been of much use.  It's hard to
get UHF to penetrate a hill or mountain.

 

We did use COFDM for our news truck microwave back to the studio location
and found QPSK with an FEC of 1/2 would get news stories in under almost all
path conditions.  When you went to higher bit rate FECs path conditions
would start to take effect.  When we started moving up to 32 and 64 QAM
things became much trickier and your path had to have evev fewer obstacles
(eg. Leaves on trees).  Changing your FEC to increase data rates and
decrease latency also had an effect. 

 

I see Terry has also responded to your initial post and I will agree 100%
with what he says.  Yes there were many reasons behind ATSC becoming the
system.

 

The comparison of ATSC and COFDM was done using a COMARK transmitter
operating at 39 KW output power into an antenna (ERP was 894 KW) located
1020 feet above sea level in NW Washington, DC.  We switched between the
COFDM and ATSC exciters and maintained the same output power for both.  MSTV
had a truck containing measurement equipment out in the field and I believe
the FCC was also out there making their own measurements.  I still remember
the folks from Sinclair down by the transmitter, with their battery powered
COFDM TV receiver, making comments about how they could receive COFDM down
there in the basement where the TX was located.  Funny thing, I could also
receive the ATSC on my receiver in the basement.

 

By the way, part of the ATSC data stream contains another service called
mobile TV.  It is basically .5 MB of data with 1 MB of error correction.  It
is intended for reception on small portable devices with small screens.  It
was not developed for home reception so HDTV receivers don't decode this in
the data stream.  The mobile stream works under severe multipath conditions,
in fact, we received the ATSC signal that contained the mobile stream
without any problem while going through the "E" street tunnel and all over
down town DC amongst the tall buildings.  We only lost lock briefly behind
the Rayburn Building and in the 3rd Street Tunnel.  Not bad for ATSC
reception on a small receiver that was made by LG Electronics.  Oh yes, the
receiver also had a broken off antenna.  Our route on this test was down
Foxhall Road, onto the Whithurst Freeway, through the E Street Tunnel, then
up to the Capital,  back behind the Rayburn building, down to and thru the
3rd Stree tunnel, onto 14th street, across the 14th Street Bridge, over
through Roslyn, up 29 to Glebe Rd., down Glebe and across Chain Bridge, up
Arizona to Lougbroro returning Foxhall.  Who says COFDM is the Cat's Meow?
Not I said the Fly!  And remember the modulation scheme is the ATSC method.

 

Bill

N4TS

 

-----Original Message-----
From: Mike O'Dell [mailto:mo at ccr.org] 
Sent: Thursday, May 16, 2013 3:06 PM
To: William Fenn
Cc: tacos at amrad.org
Subject: Re: FW: DVB-T Dongle 

 

if I remember that test correctly, COFDM kicked VSB8's ass

from DC to Baltimore and back. Much less vulnerability

to multipath and more square miles per transmitted watt 

in terms of coverage.

 

   -mo

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